In this photo taken Friday, May 3, 2013, people prepare food made from cassava flour in a market in Lagos, Nigeria. Scientists say a disease destroying entire crops of cassava has spread out of East Africa into the heart of the continent, is attacking plants as far south as Angola and now threatens to move west into Nigeria, the world's biggest producer of the potato-like root that helps feed 500 million Africans. (AP Photo/Sunday Alamba)